eBay -and expect to pay $50- $80 for it. And you do have to make sure to get one for that series of machines, as some lower end models take a similar pedal, but you do need the correct one for it to work well.
I have two 990's, but only one pedal. Machines without pedals are easy to find - so that puts the pedals in demand. I've had mine for 35 years with no problems. I do not think they go bad, they just get separated from the machine during estate sales. My MIL made sure to grab her Mom's Elna- but she did not make sure to grab anything but the machine
All the plugs/accessories were in separate case, and who knows where that ended up.